The invention relates to a method for determining the correlation between on the one hand a signal transmitted by a beacon and received at a receiver tracking said beacon and on the other hand a reconstructed signal expected to be received at said receiver from said beacon. For determining the correlation, the received signal and the reconstructed signal are shifted against each other. The invention relates in particular to a determination of the correlation for the case that the received signal comprises undesired sinusoidal modulations. The invention relates equally to a corresponding receiver and to a positioning system comprising a receiver.
A well known positioning system which is based on the evaluation of signals transmitted by beacons is GPS (Global Positioning System). The constellation in GPS consists of more than 20 satellites employed as beacons that orbit the earth. The distribution of these satellites ensure that usually between five and eight satellites are visible from any point on the earth.
Each of the satellites, which are also called space vehicles (SV), transmits two microwave carrier signals. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) Code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s, which information comprises in particular ephemeris and almanac data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The orbits calculated using ephemeris parameters are quite accurate, but the ephemeris parameters are only valid for a short time, i.e. for about 2-4 hours. The almanac data, in contrast, contain coarse orbit parameters. The orbits calculated based on almanac data are not as accurate as the orbits calculated based on ephemeris data, but they are valid for more than one week. Almanac and ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission of the code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits.
Based on the time of transmission and the measured time of arrival TOA of the signal at the receiver, the time of flight TOF required by the signal to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the general GPS time is not accurately known in the receiver. Usually, the receiver calculates the accurate time of arrival of a signal based on some initial estimate, and the more accurate the initial time estimate is, the more efficient are position and accurate time calculations. A reference GPS time can, but does not have to be provided to the receiver by a network.
The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute a position of a receiver in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
If navigation data are available on one of the receiver channels, the indication of the time of transmission comprised in a received signal can also be used in a time initialization for correcting a clock error in the receiver. In GPS, an initial time estimate is needed for the positioning. For the initial time estimate, the average propagation time of satellite signal of around 0.078 seconds is added to the time of transmission extracted from the navigation information. The result is used as initial estimate of the time of arrival of a signal, which estimate lies within around 20 ms of the accurate time of arrival. The receiver then determines for different satellites the time at which a respective signal left the satellite. Using the initial estimate of the current time, the receiver forms pseudorange measurements as the time interval during which the respective signal was propagating from the satellite to the receiver either in seconds or in meters by scaling with the speed of light. After the position of the receiver has been calculated from the determined pseudoranges, the accurate time of reception can then be calculated from standard GPS equations with an accuracy of 1 xcexcs.
However, in order to be able to make use of such a time initialization, the navigation data from a satellite signal is needed. Currently, most of the GPS receivers are designed for outdoor operations with good signal levels from satellites. Thus, only good propagation conditions ensure that the navigation data required for the described time initialization is available.
In bad propagation conditions, in contrast, it may not be possible to extract the navigation message accurately enough from received satellite signals, since a high bit-error rate and weak signal levels make a robust decoding of navigation bits impossible. Such bad propagation conditions, which are often given indoors, render the time initialization and the pseudorange measurements more difficult.
For those cases in which the standard time initialization methods can not be applied since the navigation data are noisy, the time initialization process for the receiver can be performed by a time recovery method. Some known time recovery methods are based on the cross-correlation of the tracked signal and an expected signal to define the time of transmission, as will be explained in the following.
Even in bad propagation conditions, the receiver might still be able to track the signal of a GPS satellite and to provide raw data without an evaluation of the contained bit values. Further, some information about the satellites, e.g. ephemeris and/or almanac data, might be obtained from a network, which a-priori knowledge of the GPS signal content would enable a reconstruction of certain fragments of the navigation data stream. The reconstructed data could then be utilized for the time initialization and position calculations using a cross-correlation based technique.
For the cross-correlation methods, the received raw data and expected data are shifted against each other. For each shifting position, a pointwise multiplication of the overlapping parts of the two signals is performed, taking into account different sampling rates. The multiplications for each shifting position are followed by an integration of the results. The best match between overlapping parts of the expected signal and the received raw data is assumed to be given at the shifting position resulting in the highest value in the integration.
This best match allows the determination of a time-stamp, i.e. the last bit edge transmission time, on the received signal, similar to the TOW in the conventional approach. From this time-stamp, the time of transmission of the tracked signal can then be estimated by counting corresponding epochs and chips from this time-stamp. The receiver time can be obtained by adding the average time of flight, or a more accurately estimated value of the time of flight, to the estimated time of transmission. Alternatively, GPS equations can be solved for determining the accurate receiver time. This alternative constitutes the conventional method for determining the accurate receiver time, but it requires an estimated time of transmission of signals from at least four satellites.
In cross-correlation methods, however, a problem might result from distortions in the received signals that are not wiped-off ideally by the tracking loops. In weak signal conditions, the operating mode of the tracking unit of the receiver is quite unstable, and a severe sinusoidal modulation may remain in the tracked raw data. Such uncompensated frequency distortions can result in particular from Doppler frequency shifts caused by relative receiver and satellite movements and by a clock inaccuracy.
Even a small remaining Doppler frequency shift is dangerous, since the cross-correlation based time-recovery methods integrate over several seconds, typically over 1 s to 6 s. Even if a perfect match is achieved during cross-correlation, the cross-correlation value can be very small due to the sinusoidal modulation. This becomes quite evident from the fact that after a correct alignment of the raw data and the reconstructed bits and performing an elementwise multiplication of the samples in both arrays, the data modulation is wiped-off, while the sinusoidal modulation still remains. In a real environment, the Doppler modulation is usually quite random, but in the worst case, a fixed modulation frequency will result. The cross-correlation algorithm will then integrate the complex sinusoid and output a small value. Thus, the match between the received raw data and the corresponding fragment of the expected signal will not be detected.
In a known approach for compensating sinusoidal modulations, the received and the expected signals are both separately multiplied by the shifted complex conjugate copy of the signal itself. Thereafter, the received and expected signals are cross-correlated in a conventional manner.
It is an object of the invention to provide a cross-correlation method which compensates for residual sinusoidal modulations, in particular Doppler modulations, in tracked signals of a beacon. It is further an object of the invention to provide an alternative to the known approach for compensating for sinusoidal modulations in the tracked signals of a beacon.
These objects are reached according to the invention with a method for determining the correlation between on the one hand a signal transmitted by a beacon and received at a receiver tracking said beacon and on the other hand a reconstructed signal expected to be received at the receiver from the beacon, wherein the received signal and the reconstructed signal are shifted against each other. The proposed method comprises as a first step multiplying a respective overlapping part of the received signal and the reconstructed signal for each shifting position. The multiplication can be performed in particular pointwise between the respective overlapping parts. The multiplications should take into account a possible difference in the sampling rate in the received signal and the reconstructed signal, though.
The proposed method comprises as a second step dividing the overlapping part for each shifting position into sections and integrating the products resulting in the preceding step within each section. It is to be noted that the multiplications in the preceding step do not necessarily have to be completed before these integrations start. Moreover, the division into sections can be performed already before the multiplication of the preceding step on the original samples of the received and on the reconstructed signal. It is only of relevance that the integration of the resulting products is performed separately for each section.
In a third step of the proposed method, the result of the integration of a respective first section is multiplied with a complex conjugated version of the result of the integration of a respective second section. The respective second section has a predetermined distance to the respective first section. This step is performed for a predetermined number of first sections, preferably for all sections as first section for which there exists a second section at the predetermined distance.
The products resulting in the second multiplications are then integrated in a fourth step. Finally, at least the maximum value resulting in the fourth step for the different shifting positions is determined. The shifting position with the maximum value is most probably the desired shifting position with the maximum correlation.
It is to be noted that there may be small variations in the resulting correlation values, due to which the assumed shifting position with the maximum correlation may not be quite correct. It is therefore possible, for example, to determine not only the highest correlation value but a few of the highest correlation values, in case there are doubts that the maximum correlation results in a strong enough value in a certain signal-to-noise condition. All of the associated positions can then be tried in a desired further processing.
The objects of the invention are also reached with a receiver comprising means for receiving and tracking signals from at least one beacon and processing means for realizing the proposed method.
The objects of the invention are further reached with a positioning system comprising a receiver and at least one network element of a network. This network may be a mobile communication network or any other network. The receiver comprises again means for receiving and tracking signals from at least one beacon and processing means for realizing the steps of the proposed method. In addition, the receiver comprises means for communicating with the network.
Finally, the objects of the invention are reached according to the invention with a positioning system, in which the steps of the proposed method are realized by a processing unit of the system which is external to a receiver of the system. The receiver includes in this case means for receiving and tracking signals from at least one beacon and means for providing received and tracked beacon signals to the processing unit. The processing unit can also include other functions. It can be given e.g. by a mobile station to which the receiver is connected and which is able to communicate with a mobile communication network for receiving pieces of information. It can also be given by a network element of a network, in which required pieces of information are available.
The invention proceeds from the idea that the cross-correlation technique employed for time-recovery can be modified in a way that makes it immune to residual sinusoidal modulations. Ordinary cross-correlations perform for each shifting position a multiplication of two signals followed by an integration. According to the invention, this integration is split up into two steps, between which a further operation is inserted. First, the samples resulting in the multiplication are divided into sections, and only a partial integration over the respective samples of each section is performed. The signals resulting in the first integration step are further multiplied by a complex conjugated and shifted version of itself. Only then, the resulting signals are integrated to obtain a single result for one shifting position.
It is an advantage of the invention that the residual modulation in the signals provided by a tracking loop of a receiver are compensated in the cross-correlation itself, which constitutes an alternative to the known approach. It is further an advantage of the invention that the sensitivity of the receiver is increased.
Preferred embodiments of the invention become apparent from the subclaims.
In one preferred embodiment, the length of the sections for the partial integrations is defined by the expected maximum possible frequency of undesired sinusoidal modulations, e.g. a maximum possible Doppler frequency. In addition, the number of samples per bit in the received signal should be taken into account when determining the length of the sections.
In a further preferred embodiment, the predetermined distance between the respective first and second section is equally determined based on an expected maximum possible frequency of undesired sinusoidal modulations present in the received signal. The determination can also be based on the determined length of the sections. The second section can be the section next to the first section, but also be located at a larger distance of the first section.
Advantageously, the received signal is bit-synchronized before the correlation according to the invention is performed, since this enables a correct alignment in each shifting position between the received signal and the reconstructed signal.
The method according to the invention can be employed for computing the accurate time when a received signal was transmitted by a beacon, since to the bits of the reconstructed signal, an identification may be associated which enables to determine the time at which they would have been transmitted by the beacon.
The method according to the invention can further be employed for performing a time initialization of the receiver time based on a computed time of transmission.
For the time initialization, an accurate current time of the receiver at the time of reception of the received signal can be determined as the sum of a determined accurate time of transmission and a time of flight. The time of flight can be determined based on an available position of the beacon at the accurate time of transmission of the received signal and on an available reference position of the receiver.
Alternatively, in case the receiver receives signals from at least four beacons and determines the accurate time of transmission of each of these signals, an accurate current time of the receiver at the time of reception of the received signals can be determined by conventional GPS equations. It is possible to employ different methods for determining the time of transmission of signals in different channels. Thus, it is only required that the time of transmission of the signal from one of the at least four beacons is determined according to the method of the invention as basis for the GPS equations.
It is understood that the employed expression xe2x80x9caccurate timexe2x80x9d does not refer to an absolute accuracy but only to a quite high accuracy.
In case the receiver is able to communicate with a network, the receiver may receive various information as a basis for the calculations according to the invention. It is to be noted that the receiver can be able to communicate with the network either directly or indirectly, in the case of a mobile communication network for instance via some mobile station. A network may provide a receiver for example with a reference time for the receiver, with a maximum error of this reference time, with a reference position of the receiver and with position information for at least one beacon. The position information can include in particular ephemeris data and/or almanac data for at least one beacon. In some situations, only ephemeris data, only almanac data or both might be available at a network, and only the available data can be provided. As mentioned above, the network providing assistance data can be a mobile communication network, but it can also be any other kind of network which is capable of providing assistance data via a network element, e.g. via a DGPS (Differential Global Positioning system) station.
In an advantageous embodiment of the positioning system according to the invention including a network element of a network, the network element comprises therefore means for receiving and tracking signals from at least one beacon, and moreover means for providing the receiver with at least one of the above mentioned pieces of information. It is to be noted that in case pieces of information are provided by a network, not all of the listed pieces of information have to be provided. The maximum error of a time reference could be specified for example by requirements to the reference time in a standard or a system specification. In this case, there would be no necessity to communicate the maximum error to the receiver, as it is known.
Each of the mentioned data may alternatively be stored in the receiver or be provided by some algorithm in the receiver or a connected processing unit, e.g. another time-recovery algorithm providing an estimate of the current time and the maximum possible error in this estimate. Thus, a receiver according to the invention can also operate independently of assistance data from a network.
If some required information is missing at the processing means, a signal can only be partially reconstructed. But with some care, the method according to the invention can still be applied. Not reconstructed bits could be replaced e.g. by 0, while reconstructed ones will be set to xc2x11. A control can be maintained in the receiver during each cross-correlation by monitoring the number of xe2x80x9cnot reconstructedxe2x80x9d bits having a value of xe2x80x9c0xe2x80x9d. If that number is not big, the cross-correlation is performed, but if the reconstructed array is found to be almost empty, this fragment is not used and the receiver will wait for a more favorable moment. Since different phases are compared by sliding and cross-correlating, the cross-correlation peak value depends on the number of xe2x80x9cunknownxe2x80x9d bits at the given stage, which number changes from one sliding position to the next. A kind of scaling may be used to normalize properly, so that the method according to the invention still works normally with some unknown bits.
Preferably, though not necessarily, the modified cross-correlation according to the invention is implemented as software.
The invention can be employed in particular in the current GPS system, but equally in future extended GPS systems with new signals and in other similar beacon based positioning systems such as Galileo. It can further be employed in any system in which a cross-correlation has to be performed with beacon signals received at a receiver.
The beacon can be in particular, though not exclusively, a satellite or a base station of a mobile communication network.
Preferably, though not necessarily, the receiver is a GPS receiver and the beacon is a GPS space vehicle.